1. Field of the Invention
The present invention relates to a method of forming a micromechanical structure, and more particularly, to a method of preventing peeling between sacrificial silicon layers in the microelectromechanical structure (MEMS) process.
2. Brief Description of the Related Art
The use of silane (SiH4) as a main reaction gas to deposit sacrificial silicon layers is a common step in the manufacture of semiconductor devices and MEMS. MEMS have found applications in inertial measurement, pressure sensing, thermal measurement, micro-fluidics, optics, and radio frequency communications, and the range of applications for these structures continues to grow. One example of such a structure is a reflective spatial light modulator, which is a device consisting of a planar array of electrostatically deflectable mirrors, each microscopic in size. The device is used as a micro-display system for high resolution and large screen projection. The sacrificial silicon layer in such a device is the layer over which the mirror material is deposited. Once the mirror structure is formed, the sacrificial silicon layer is removed to leave gaps below the mirrors and microhinge along one edge of each mirror to join the mirror to the remainder of the structure. The gap and the microhinge provide the mirror with the freedom of movement needed for its deflection. Devices of this type are described in, for example, U.S. Pat. Nos. 6,356,378, 6,396,619 and 6,529,310.
The success of a manufacturing procedure for structures involving sacrificial silicon layers depends on the interface adhesion therebetween. The thickness and lateral dimensions of the layers, and in the case of the deflectable mirror structures, the width of the gap and the integrity of the microhinges, are all critical to achieve uniform microstructure properties and a high yield of defect-free product. One of the critical factors is the interface quality between the sacrificial silicon layers. Performance, uniformity and yield can all be improved with increases in the interface adhesion between the sacrificial silicon layers. Hereinafter, parts of a traditional micromirror structure process will be described, with reference to FIGS. 1A and 1B.
In FIG. 1A, a light transmissive glass substrate 100 is provided. A first sacrificial silicon layer 110 is deposited on the substrate 100. The first sacrificial silicon layer 110 is an amorphous silicon or crystalline silicon layer. A mirror plate 120 is then defined on part of the first sacrificial silicon layer 110. The mirror plate 120 can be a metal plate.
Referring to FIG. 1A, unwanted remnants (or byproducts) from the fabrication of the mirror plate 120 are then removed by argon (Ar) plasma cleaning (or sputtering) 130. Though effective, the Ar plasma cleaning 130 leaves remnant silicon dangling bonds on the surface of the first sacrificial silicon layer 110 exposing it to environmental and atmospheric impurities. The impurities again attach to the silicon dangling bonds on the surface of the first sacrificial silicon layer 110.
In FIG. 1B, a second sacrificial silicon layer 140 is deposited on the mirror plate 120 and the first sacrificial silicon layer 110. The second sacrificial silicon layer 110 is an amorphous silicon or crystalline silicon layer. It is noted that, since the surface of the first sacrificial silicon layer 110 has impurities, robust covalent (Si—Si) bonds at the interface between the first and second sacrificial silicon layers 110 and 140 cannot be thoroughly formed. That is, peeling 150 frequently occurs between the sacrificial silicon layers 110 and 140 after depositing the second sacrificial silicon layer 140. The peelings 150 cause the surface 141 on the second sacrificial silicon layer 140 to be rough, thereby affecting the subsequent photolithography and etching. In addition, the peeling issue will worsen with subsequent repeated thermal processes, thereby generating particles which contaminate other processing tools.
In U.S. Pat. No. 5,835,256, Huibers et al disclose a deflectable spatial light modulator (SLM). The method describes formation of silicon nitride or silicon dioxide mirror elements and the underlying polysilicon sacrificial layer serving as a support to be removed in subsequent etching. Nevertheless, the method does not eliminate the peeling issue in the sacrificial silicon layer.
In U.S. Pat. No. 6,396,619, Huibers et al disclose a deflectable spatial light modulator (SLM). The sacrificial material layer can be silicon or polymer. Nevertheless, the method does not teach how to solve the peeling issue of the sacrificial silicon layer.
In U.S. Pat. No. 6,290,864, Patel et al disclose a procedure of etching sacrificial silicon layers for the manufacture of MEMS. The method, utilizing noble gas fluorides or halogen fluorides as etchant gases, exhibits greater selectivity toward the silicon portion relative to other portions of the microstructure by incorporating non-etchant gaseous additives in the etchant gas. Nevertheless, this method does not eliminate peeling in the sacrificial silicon layer.